I keep my Droid in my pockets and have a Seido Innocase, but still get annoyed at the tendency of the thing to partially slide open. To deal with that, I bought a "G1" slipcase off of eBay for under $4.00. Search for something like "OEM G1 case." and you'll find it from various sellers. I believe it is the same or similar to the slipcase that came with the HTC G1.
My Innocase is now partially held onto my phone with small pieces of tape that is folded over and inserted between the phone and the case. Two clips on the screen part and two clips on the body part of the case have broken causing the need for tape.
The body part of the OtterBox case looks significantly stronger than the Seido one, but two friends with OtterBoxes have broken top cases.
I'm fairly clumsy and also have kids, so I like keeping my phone in a case. Even though I like physical keyboards, my next phone will probably be without one so I can use something like an OtterBox Defender case.